One of the important issues in wireless communication systems is capacity of the system. Smart antenna technology has been developed to increase wireless communication system capacity. Smart antennas are currently used in base stations, access points, and WTRUs. One form of smart antenna technology is the use of multiple radiating elements in one or more antennas to generate a plurality of directional beams. With this form of smart antenna, use of the directional beam or beams with the best quality reduces the amount of transmit power needed, usually resulting in increased system capacity.
In mobile communication systems, WTRUs typically monitor quality, such as signal-to-interference ratio (SIR), of the cell(s) currently serving the WTRU as well as neighboring cells. In WTRUs employing smart antenna techniques which generate a plurality of beams, the WTRUs would need to monitor the quality of the plurality of beams for all of these cells (or a subset of these cells).
Hereafter, the terminology “active beam” refers to a beam that a WTRU uses for its data transmission and reception, and the terminology “serving base station” refers to a base station currently communicating with the WTRU. The terminology “current beam” refers to the beam currently being formed by the element(s) of the antenna(s). In order to measure quality (such as SIR) on channels that correspond to non-active beams, the WTRU must switch its current beam to the non-active beam and observe the channel for some time. This time period is referred to as “dwell time”, T_DWELL. Once the dwell time expires, the WTRU switches the current beam back to the original active beam for normal communication with the serving base station(s).
In the prior art, in order to measure the signal quality on inactive beams on multiple base stations, the WTRU switches its current beam to the inactive beams for each of those base stations for a period of time. For example, if a WTRU uses a smart antenna which is configured to generate three beams (a left beam, an omni-directional beam and a right beam), and if the right beam is an active beam and the WTRU has to measure SIRs to three base stations (BS-1, BS-2, and BS-3) using the left beam, the WTRU first switches the current beam from the right beam to the left beam for T_DWELL to measure the SIR to BS-1. During this time, for a CDMA system for example, the WTRU despreads the received signal using the known pilot (or other) signal transmitted from BS-1, and the despread values are used to estimate the SIR to BS-1. In order to measure the SIR to BS-2, the WTRU again switches the current beam to the left beam for another T_DWELL, and receives signals and despreads the received signal using the known pilot (or other) signal transmitted from BS-2. The despread values are then used to estimate the SIR to BS-2. Similarly, in order to measure the SIR to BS-3, the WTRU has to switch the current beam to the left beam again for another dwell time. Therefore, in this example, the WTRU must stay on the left beam for 3*T_DWELL to measure the SIRs on the left beam for all three base stations.
Data reception is degraded during the dwell time since the WTRU operates based on the assumption that the channel it sees corresponds to the active beam. In the foregoing example, data reception is interrupted for 3*T_DWELL. More generally, in accordance with the prior art, a WTRU must switch a beam for N*T_DWELL, to measure the SIR to N base stations on an inactive beam. Since data can be continuously transmitted to the WTRU, it is necessary to keep the dwell time as short as possible.
It is noted that the operations for despreading above (or other means for correlating the received signal with a known transmit signal) are done in real-time using the correlation resources in the mobile receiver (hardware blocks and/or software blocks in a microprocessor or DSP).